It is often desirable to obtain measurements of selected characteristics of sheet materials during manufacture. Although various properties of sheet materials can be detected by off-line laboratory testing, such tests often are not practical because of the time required for sample acquisition and analysis. Also, laboratory testing has the shortcoming that samples obtained for testing may not accurately represent sheet material that has been produced.
To overcome the drawbacks of laboratory testing of sheet materials, various sensor systems have been used for detecting sheet properties “on-line,” i.e., on a sheet-making machine while it is operating. Typically, on-line sensor devices are operated to periodically traverse, or “scan,” traveling webs of flat sheet material during manufacture. Scanning usually is done in the cross direction, i.e., in the direction perpendicular to the direction of sheet travel. Depending upon the sheet-making operation, cross-directional distances can range up to about 10 meters or more.
A wide variety of scanning sensor devices has been used for on-line measurements of sheet materials. In the papermaking industry, for example, spectrometric scanning sensors have been employed for on-line detection of characteristic sheet materials. A spectrometric scanning system typically includes a stationary frame having a pair of stiff upright end members that stand on a factory floor for supporting a pair of parallel, solid guide members or rails that extends horizontally across the opposite faces of a traveling sheet. The guide members often are quite massive. Adjustment systems may be employed to straighten the guide members independent of the upright end support structure to compensate for non-straightness of the guide member or weight deflection. A motor driven carriage is mounted to travel on the guide members. The carriage is connected to a drive system to be driven back and forth across sheet. The scanning sensor system also includes a scanning head that is mounted on the carriage member. The scanning head contains the detection components. For example, in the case of a spectrometric analyzer, the scanner head can include a source of infrared light, collimating and beam-splitting mirrors, and photosensitive detector.
In order to monitor and control a flat sheet manufacturing process, it is desirable and sometimes necessary to position across directional scanning sensors at a number of strategic locations throughout the process. However, this is often very difficult because of space restrictions in a facility. In particular, since a sensor is ordinarily influenced in some manner by the distance between it and the object being measured, the supporting guide members, e.g., beam, which span the width of the process, have typically been designed with very high structural inertia to counter the deflection caused by sag and vibration. To generate the required inertia levels, however, the dimensions of the guide members often become so large that they cannot fit within the space allotted. While sag can be compensated for with manufacturing methods, long and bulky, low inertia, guide members are still susceptible to resonant vibration deflection. Thermal distortion of the structures caused by uneven heating often times must be dealt with no matter how strongly the guide members are built.